This invention relates generally to hermetic refrigeration compressors, and more particularly to small high-efficiency compressors utilizing rolling element bearings.
Household appliances, such as refrigerators and food freezers, almost universally use small hermetic compressors, many of which employ a single reciprocating piston driven by a crankshaft and connecting rod and driven by a two-pole electric motor at a nominal speed of 3600 rpm. The particular nature of this application for the compressor results in many problems in trying to increase the overall energy efficiency of the refrigerator unit. While a considerable increase in efficiency can be obtained by a more efficient insulation of the cabinet and sizing and design of condensers and evaporators, the hermetic compressor is still an area where substantial increases in operating efficiency can be obtained. The refrigeration compressor tends to be inefficient for a number of reasons, including such factors as the requirement of far more torque under start-up conditions than during normal run conditions and the requirement of a motor size that tends to become less efficient and operates at a lower power factor under normal run conditions in order to operate and give proper performance under other conditions such as start-up and low voltage. Likewise, because such compressors tend to be of relatively low displacement as compared to compressors used in other areas such as air conditioning and the like, where higher outputs are required, such compressors tend to have a lower mechanical efficiency, and therefore proportionally greater start-up and running friction in the bearings.
For reasons of compactness of size, as well as obtaining proper lubrication and physical simplicity, such single piston compressors utilize a crankshaft rotating about a vertical axis with a horizontally moving piston that may be located either at the top or the bottom of the compressor housing, and therefore either above or below the motor stator. With this arrangement, the crankshaft is usually supported on two axially spaced bearings which are positioned in regard to the forces acting on the crank from the motor, the location of the stator, and the position of the eccentric or crank pin which, through a connecting rod, reciprocates the piston. Because the crankshaft is vertical, it requires a form of thrust bearing to take the vertical weight of the crankshaft and motor rotor. To provide lubrication, it is conventional to have the lower end of the crankshaft extend into the body of lubricating oil in the bottom and, by virtue of having vertical passages offset from the axis of rotation, centrifugal force is used to cause the oil to flow upwardly in the crankshaft and provide suitable lubrication for all of the bearings and the piston and connecting rod.
The bearings for these compressors have generally been of the plain sleeve type because such bearings are low in cost, give long life, and are relatively quiet when the compressor is in operation. However, such bearings tend to produce high friction loads under certain operating conditions and during start-up, particularly when the compressor has not been operating for a long time and there may be an insufficient amount of lubricating oil on the bearing surfaces. When consideration has been given to the use of ball or roller bearings in these small compressors, problems have been encountered because of the high cost of such bearings as compared to plain sleeve bearings and the increased space requirements for their bearings may increase the overall compressor size. While such bearings reduce both the static and dynamic friction under start-up and running conditions, their life tends to be erratic because of the high degree of precision required in mounting the bearings, and such bearings tend to be somewhat noisy in operation. As a result of these factors, such rolling element bearings have not been used in small hermetic compressors of the type used for household refrigerators and food freezers.